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In this article we will discuss about the structure of a cell wall in plants. This will also help you to draw the structure and diagram of cell wall in plants.
Gross Structure of Cell Wall:
A plant cell may consist of either primary wall only or the cell may contain both primary and secondary walls (Fig. 3.1).
i. Primary Wall:
The first formed wall of a cell is the primary wall. It usually grows in surface area.
Primary wall is about 0.1 µm thick normally, but the walls of collenchyma may be 10 µm thick. The cuticularized epidermal cells may be thicker. Thick cellulosic primary cell walls are observed in the endosperm cells of Phoenix dactylifera, Strychnos nux-vomica etc. Thick primary wall, in contrast to secondary wall, is plastic and reversible.
The meristem, cambium cell, parenchyma, collenchyma, root hairs etc. possess primary walls. The primary wall of cambium cells shows seasonal changes in thickness and the thick wall of endosperm cells of Phoenix, Strychnos etc. are digested during germination.
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The components of primary wall are deposited on both the sides of middle lamella, which initially creates a boundary between two nuclei at the end of nuclear division.
As more wall materials are deposited and growth occurs, the distinction between middle lamella and primary walls is obliterated. At maturity the middle lamella and the two primary walls of adjacent cells become confluent and it becomes difficult to define a precise boundary between them.
ii. Secondary Wall:
A secondary wall is one whose polysaccharide components are deposited over the primary wall. In contrast to primary wall, secondary wall is formed after the cessation of surface growth. Therefore, it causes growth in thickness only.
Since the secondary wall materials are deposited outside the plasma membrane, it is present internal to primary wall surrounding the cell lumen. In addition to cellulose, hemicelluloses and other polysaccharides, the secondary wall contains lignin.
When lignification occurs, it may begin at either the primary wall or middle lamella. In the deposition of secondary wall materials layering can be observed. The lignified tracheid and fibre show three layers in their secondary wall the outer layer (S1) the central layer (S2) and the inner layer (S3), among which the central (S2) is the thickest. The S1 and S3 layers lie adjacent to primary wall and cell lumen respectively.
Cells with secondary wall consist of five layers a three layered secondary wall, the primary wall and the middle lamella. In some cells, such as primary xylem, the secondary thickening materials are laid down in such a way that various patterns are formed on the cell wall, e.g. annular, spiral, reticulate, scalariform and pitted. In contrast to primary wall, secondary walls are irreversible and provide mechanical strength. Usually secondary walls are present in those cells, which are devoid of active protoplast.
iii. Middle Lamella:
This layer lies in between the two primary walls of adjacent cells. It is the cementing material that fastens cells to their neighbours; therefore, the cells are held together by their primary walls. The middle lamella is very thin (less than 30nm thick) and thickest at the cell corners. Pectic substances, possibly in association with calcium and others compose the middle lamella.
Middle lamella originates at the end of nuclear division and creates a new boundary between the two nuclei, which had previously shared a common cytoplasm. As growth continues the primary wall materials are laid on either side of middle lamella. At maturity, the layers are indistinguishable and it becomes difficult to define a precise boundary between them.
